Welcome to the forum David. There's lots of threads on here that you can research to help you. Keep in mind some threads may not have working pictures due to those being hosted by Photobucket that no longer work.
Judging from the pics you posted, it looks like you'll need to do a full replacement of the core and transom. From what I remember, there shouldn't be plywood stringers in your boat. Hydrostream used a pre-formed fiberglass keelbox about 6-7 inches wide that ran down the pad. (Basically a square U-channel that is upside down).
You'll need to remove that as well to get to the balsa core in the pad.
Start a worklog in the Restorations section. Let's see your work and we'll be able to answer any questions you have as you progress on the rebuild. Ron (alaskastreamin) is a wealth of knowledge so don't be afraid to pick his brain every chance you get.
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